Add preview command

Author: agualis

README.md

@@ -4,8 +4,18 @@ Balancer documentation built using Vuepress and extension of a custom theme. Ful
 
 ## Local Setup
 
-```
+```bash
 npm install
 npm run build-theme
 npm run dev
 ```
+
+## Build preview
+
+Useful to test only-production features like metadata
+
+```bash
+npm run build-theme
+npm run build
+npm run build:preview
+```